Cyanate ester with urea latent cure accelerator

ABSTRACT

Urea compounds having a plurality of substituents selected from hydrogen, substituted hydrocarbyl radicals and unsubstituted hydrocarbyl radicals are good latent cure accelerators for thermosetting cyanate resin formulations. The resulting cyanate resin formulations are useful as coatings, adhesives and as impregnating resins, particularly in applications where extended storage at or near room temperature is desired.

BACKGROUND OF THE INVENTION

This invention relates to thermosetting resins and more particularly tothermosetting cyanate esters. Still more particularly, this inventionrelates to the use of urea compounds as latent cure accelerators forcyanate esters, and to thermosetting cyanate ester formulationscomprising cyanate esters and urea compounds.

Cyanate esters are well-known in the art and widely used in formulatingadhesives, binders, coatings and impregnants. Such formulations may alsoinclude oligomeric compounds with reactive cyanate ester functionalityas well as a variety of other coreactants such as epoxy resins in orderto reduce costs and to modify properties such as toughness, moisturesensitivity and thermal behavior in the resulting thermoset materials.

Cyanate esters may generally be cured merely by heating. Catalysts usedto promote curing under milder conditions have included Lewis acids suchas aluminum chloride, ferric chloride and the like, mineral acids suchas hydrochloric acid, salts such as sodium acetate, potassiumthiocyanate and the like, phenolic compounds and bases such as sodiummethoxide, pyridine, triethyl amine and the like. Metal chelates such ascopper, zinc or ferric acetylacetonates have been reported as beingcapable of promoting a smooth, controllable cure rate at moderatetemperatures. Such catalysts are said to be generally less moisturesensitive, and possibly less hazardous than many of the catalyst systemsavailable for cyanate esters.

Many of the prior art catalysts are highly active and may even promoterapid curing at room temperature in many cyanate ester formulations. Thestorage stability of cyanate ester materials and formulations based onsuch catalysts may therefore be brief, making the formulations difficultto use for many applications by requiring storage conditions that may bedifficult or impractical to achieve. The more stable cyanate esterformulations, those based on the less-active prior art catalysts, may bemore difficult to cure adequately even when extended cure cycles areused. Extended cure times, particularly at elevated temperatures,increase the cost of production and may cause damage to substrates aswell as to other components of the formulation. In addition,insufficient cure levels tend to result in brittle materials having anincreased sensitivity toward moisture. Cyanate ester cure acceleratorshaving little or no catalyst activity at or near room temperature and ahigh degree of activity at moderately elevated temperatures are thusneeded. Such accelerators are termed latent cure accelerators, and maybe used to provide storage-stable cyanate ester formulations that arerapidly and completely cured at moderate temperatures.

Some of the presently available catalysts exhibit a degree of latentcuring character or latency when used in combination with some cyanateresins. However, such catalysts are few in number. The uses of cyanateester formulations by the coatings, adhesives and laminating artsencompass a great variety of applications. The curing conditionsrequired by these applications will vary widely, and the latent curingbehavior needed for some applications may be measured in hours, whileothers may require stability for days or even weeks at room temperature.Moreover, the residues characteristic of some catalysts may not beacceptable for particular applications and end uses. Thus there is acontinuing need for a greater variety of cure catalysts and latent cureaccelerators, in order to allow the resin formulator to modify thecuring behavior and storage characteristics of cyanate ester-based resinformulations, thereby becoming better able to meet the demands of theseindustries.

SUMMARY OF THE INVENTION

N-substituted urea compounds are effective latent cure accelerators forcyanate esters. Formulations comprising such urea compounds exhibit goodstorage stability at room temperature and rapidly reach a high degree ofcure at moderately elevated temperatures. The formulations areparticularly useful in coatings, adhesive and impregnant applicationswhere extended storage of coated or impregnated substrates in an uncuredstate may be desirable.

DETAILED DESCRIPTION

The urea compounds useful as cure accelerators in the practice of thisinvention are urea compounds having a plurality of N substituents. Thecompounds may be further represented by the structural formula RR¹--N--CO--N--R² R³, wherein R and R² are independently selected fromhydrogen and organo radicals, and R¹ and R³ are independently selectedorgano radicals. The organo radicals may be either substituted orunsubstituted aliphatic and aromatic hydrocarbyl radicals, includingthose selected from, for example, C₁ to C₆ alkyl radicals, aralkylradicals, aryl radicals and the like, and R and R¹ may join together toform a cycloalkylene radical. The hydrocarbyl radicals may be furthersubstituted with any of a variety of groups including halogen or thelike which are inert and nonreactive toward the remaining components ofthe resin formulations. Examples of such urea compounds include thealkyl aryl ureas and aryl ureas such as 1,1-dimethyl-3phenyl urea,1,1-dimethyl-3-(4-chlorophenyl) urea,1,1-dimethyl-3-(3,4-dichlorophenyl) urea, 1,3-diphenyl urea,1-(4-chlorophenyl)-3 -(3,4-dichlorophenyl) urea and the like. Alsouseful are urea compounds having a plurality of urea functional groups,including for example the reaction products of alkyl amines, alkylenediamines and dialkylene triamines with aryl isocyanates anddiisocyanates, such as those urea compounds shown for example in U.S.Pat. Nos. 3,386,955 and 4,594,373.

The cyanate esters useful in preparing formulations curable with ureacompounds according to the teachings of this invention are arylcompounds having a plurality of cyanate ester groups per molecule, andmay be generally represented by the formula Ar(OCN)_(m) wherein m is aninteger of from 2 to 5 and Ar is an aromatic radical. The aromaticradical Ar will contain at least 6 carbon atoms, and may be derived, forexample, from aromatic hydrocarbons such as benzene, biphenyl,naphthalene, anthracene, pyrene or the like. The aromatic radical Ar mayalso be derived from a polynuclear aromatic hydrocarbon in which atleast two aromatic rings are attached to each other through a bridginggroup. Also included are aromatic radicals derived from novolak-typephenolic resins, i.e., the cyanate esters of these phenolic resins. Thearomatic radical Ar may also contain further ring-attached, nonreactivesubstituents.

Useful cyanate esters may include, for example, 1,3-dicyanatobenzene;1,4-dicyanatobenzene; 1,3,5-tricyanatobenzene; 1,3-, 1,4-, 1,6-, 1,8-,2,6- or 2,7-dicyanatonaphthalene; 1,3,6-tricyanatonaphthalene;4,4'-dicyanatobiphenyl; bis(4-cyanatophenyl)methane and3,3',5,5'-tetramethyl bis(4-cyanatophenyl) methane;2,2-bis(3,5-dichloro-4-cyanatophenyl)propane;2,2-bis(3,5-dibromo-4-dicyanatophenyl)propane;bis(4-cyanatophenyl)ether; bis(4-cyanatophenyl)sulfide;2,2-bis(4-cyanatophenyl)propane; tris(4-cyanatophenyl)phosphite;tris(4-cyanatophenyl)phosphate; bis(3-chloro-4-cyanatophenyl)methane;cyanated novolak; cyanated bisphenol-terminated polycarbonate or otherthermoplastic oligomer; and mixtures thereof. Also included are cyanatesof poly(alkenylphenol) compounds disclosed in U.S. Pat. No. 4,477,629,cyanates from bisphenols of dicyclopentadiene which are disclosed in,for example, U.S. Pat. No. 4,528,366, the cyanates disclosed in BritishPat. No. 1,305,702, and the cyanates disclosed in PCT publishedapplication No. WO 85/02184. These and a wide variety of other cyanateesters are widely known in the art and many are commercially available.

The cyanate esters may be used singly or as mixtures. The cyanate estersmay also be used in the form of a prepolymer, made by heating apolyfunctional cyanate monomer at a temperature of 130° to 220° C. for aperiod of 0.1 to 15 hours, oligomerizing the cyanate ester andincreasing the molecular weight. Also useful are mixtures of theprepolymer with monomeric cyanate esters, and many of the commerciallyavailable cyanate esters are such mixtures of cyanate monomers andprepolymers.

In general, the thermosetting compositions of this invention willcomprise from 0.5 pbw to 12 pbw of the urea compound per 100 pbw of thecyanate ester. The specific level employed will depend in part upon theparticular cyanate ester and urea compounds employed.

The compositions of this invention may further comprise additionalpolymerizable, curable components, such as, for example, epoxy resins,bismaleimide resins and the like.

Epoxy resins useful as further components in the practice of thisinvention include any of the great variety of polyfunctional epoxyresins widely known and readily available from commercial sources. Amongthese are the polyglycidyl derivatives of phenolic compounds, such asthose available commercially under the trade names such as Epon 828,Epon 1001, Epon 1009 and Epon 1031 from Shell Chemical Co., DER 331, DER332, DER 334 and DER 542 from Dow Chemical Co., and BREN-S from NipponKayaku, Japan. Other suitable epoxy resins include polyepoxides preparedfrom polyols and the like and polyglycidyl derivatives ofphenol-formaldehyde novolaks. The latter are commercially available asDEN 431, DEN 438, and DEN 439 from Dow Chemical Company. Cresol analogsare also available as ECN 1235, ECN 1273, and ECN 1299 from Ciba-GeigyCorporation. SU-8 is a Bis-A epoxy novolak from Interez, Inc.Polyglycidyl adducts of amines, aminoalcohols and polycarboxylic acidsare also useful in the practice of this invention. Commerciallyavailable resins of this type include Glyamine 135, Glyamine 125, andGlyamine 115 from F.I.C. Corporation, Araldite MY-720, Araldite 0500,and Araldite 0510 from Ciba-Geigy Corporation and PGA-X and PGA-C fromThe Sherwin-Williams Co.

Also suitable are epoxy-terminated thermoplastic polymers such as theepoxy-terminated polysulfones disclosed in U.S. Pat. No. 4,448,948.

The bismaleimides that may be used as further components in the presentinvention are organic compounds containing two maleimide groups and areprepared generally from maleic anhydride and diamines. The preferredbismaleimides are derived from aromatic diamines and most preferred arethose that include a polynuclear aromatic radical. Examples of suchbismaleimides include 2,2-bis(4-aminophenoxy-4-phenyl)propanebismaleimide, 4,4'-bis(3-aminophenoxy)diphenyl sulfone bismaleimide,1,4-bis(3-aminophenyl isopropylidene)benzene bismaleimide andbis(4-aminophenyl)methane bismaleimide. The bismaleimides may be usedsingly or as mixtures. Bismaleimides may be prepared by a number ofwell-known methods from maleic anhydride and diamines, and a great manyare readily available from commercial sources.

The curable cyanate ester formulations of this invention may include anyof a variety of additional modifying components to meet the requirementspeculiar to the intended use. Where the added resins are also curableand thermosetting, it may be necessary and desirable to include curingagents for such resins. Thermoplastics such as polysulfones, poly(arylethers), aromatic polyesters, polyamides, and the like, and rubberymodifiers such as silicone rubbers, diene rubbers, acrylic rubbers andrubbery polyesters, and particularly those thermoplastics and modifiersthat are miscible with cyanate esters, all of which are well-known andfrequently used in the resin formulating art, may be added to provideimproved toughness. Organic and inorganic fillers and reinforcing fibersmay also be included, as may pigments, dyes, lubricants, thickeners,stabilizers and the like as is commonly practiced in the art. Thecurable cyanate ester formulations of this invention may be particularlyuseful as matrix resins and, when combined with reinforcing fibers suchas glass fiber, carbon fiber, graphite fiber and the like, in chopped orcontinuous form or in the form of either woven or non-woven textilefabric or mat, the compositions may be used for the production ofprepreg, fiber-reinforced laminates, composites and the like.

The practice of this invention will be better understood from aconsideration of the following illustrative examples. In the followingexamples, the components and test procedures used include:

Cyanate Esters

Cyanate Ester A. Prepolymer of bisphenol A dicyanate, obtained as RDX80352 from Interez, Inc.

Cyanate Ester B. A polycyanate of a polyphenol adduct ofdicyclopentadiene, obtained as XU71787 from Dow Chemical Company.

Cyanate Ester C. A prepolymer of bisphenol A dicyanate containing 10percent by weight of 4,4'-methylene dianiline bismaleimide, obtained asBT2160 resin from Mitsubishi Gas Chemical Company.

Epoxy Resins

Epoxy DEN 431. Epoxidized phenol-formaldehyde novolac with a weight perepoxy equivalent of 176 g, from Dow Chemical Company.

Epoxy Epiclon 830. A bisphenol F epoxy resin with a weight per epoxyequivalent of 170 g, obtained from Dianippon Ink Company.

Thermoplastic

Polysulfone PSF. Polyarylene ether of bisphenol A and dichlorodiphenylsulfone, Mn=24,000, from Amoco Performance Products, Inc.

Gel Time Procedure:

Gel times were measured by placing a small portion (˜0.1 g) of the resinmixture between two circular microscope slips on the heated stage of aFisher-Johns melting point apparatus, preheated to 350° F. Periodicallythe top glass slip was poked with a wooden stiple to see if the resinwas still fluid. The gel time was taken as the time when the resin firstfailed to flow under the influence of pressure. The gel times were anaverage of from two to six determinations.

EXAMPLES 1-11

A 4 oz glass jar, charged with 50 g of the cyanate ester was heated inan oil bath at 80° to 100° C. The contents of the jar were rapidlystirred as the urea compound was added, stirred for an additional 3 min.and then removed from the bath. The gel time at 350° F. was thenmeasured.

Table I summarizes gel times for various cyanate esters and acceleratormixtures, and for the control Examples A-D, determined without addedurea compound. Typically at the 4 phr level, the accelerators of thisinvention reduce the gel times by 1/2 to 3/4 of the gel time values forthe unaccelerated cyanate esters.

                  TABLE I                                                         ______________________________________                                        Gel Times at 350° F.                                                   Ex.  Urea                      Gel Time                                       No.  Cmpd (4 pbw)              (min.)                                         ______________________________________                                        Cyanate Ester A (100 pbw)                                                     1    1,1-dimethyl-3-phenyl urea                                                                              17                                             2    3-(4-chlorophenyl)-1,1-dimethyl urea                                                                    14                                             3    3-(3,4-dichlorophenyl)-1,1-dimethyl urea                                                                14                                             4    1,3-diphenyl urea         23                                             5    2,4-bis(N,N--dimethylureido) toluene                                                                    10                                             A    None                      >50                                            Cyanate Ester B (100 pbw)                                                     6    1,1-dimethyl-3-phenyl urea                                                                              29                                             B    None                      70                                             Cyanate Ester C (100 pbw)                                                     7    1,1-dimethyl-3-phenyl urea                                                                              10                                             C    None                      46                                             Cyanate Ester A (80 pbw) and Epoxy DEN 431 (20 pbw)                           8    1,1-dimethyl-3-phenyl urea                                                                               8                                             9    3-(4-chlorophenyl)-1,1-dimethyl urea                                                                    12                                             10   3-(3,4-dichlorophenyl)-1,1-dimethyl urea                                                                10                                             11   1-(4-chlorophenyl)-3-(3,4-dichlorophenyl) urea                                                          10                                             D    None                      22                                             ______________________________________                                    

It will be apparent from a comparison of the gel times for the variouscyanate compositions presented in Table I that the gel times at 350° F.are markedly shortened when an organo-substituted urea compound is addedthereto.

EXAMPLES 12 through 16 and Controls E through G

A series of unreinforced castings were prepared from various cyanateester/accelerator formulations.

In a typical procedure, a 4 oz glass jar was charged with 48 g ofcyanate ester and heated in an oil bath at 80° to 100° C. After thematerial became fluid, it was stirred while 2.0 g of a urea acceleratorwere added. Stirring was continued for three minutes, after which aportion of the mixture was poured into a glass mold with dimensions of1/8"×3"×4".

The formulation was cured in an oven using the following cure schedule:

Heat from 77° F. to 248° F. at 3° F./min

Hold 2 hr at 248° F.

Heat from 248° F. to 350° F. at 3° F./min

Hold 2 hr at 350° F.

Cool to 77° F. at 3° F./min

Test coupons with nominal dimensions of 1/8"×0.4"×3" were cut from thecured castings to determine the glass transition temperature using aDuPont 982 dynamic mechanical analyzer. The glass transition temperaturewas taken as the maximum in the loss modulus peak. The heating rate forall glass transition temperature measurements was 10° C./min.

EXAMPLE 17

A 250 ml, 3-necked flash equipped with a paddle stirrer, thermometer,inlet and outlet for inert gas, and heated in an oil bath was chargedwith 18 g of Epiclon 830 and 8 g of powdered polysulfone. The mixturewas heated and stirred for 1 hr at 140° C. as the polysulfone dissolved.To this stirred solution at 140° C. was added 72 g of warm (100° C.)Cyanate Ester B over a 5 minute period. As soon as the addition wascomplete, the mixture was stirred for 5 more minutes as it cooled to100° C. To this solution was added 2.0 g of 1,1-dimethyl-3-phenyl urea.The mixture was stirred at 100° C. for 3 minutes. Then the mixture waspoured into a glass mold having a cavity with dimensions of 1/8"×4"×3"and cured using the following cure schedule:

Heat from 77° F. to 350° F. at 1.1° F./min.

Hold 2 hr. at 350° F.

Cool to 77° F. at approx. 2.2° F./min.

The cured casting was hard and clear. A coupon with approximatedimensions of 0.4"×3.0"×1/8" was cut from the casting and was used tomeasure the glass transition temperature on a DuPont DMA (scan rate: 10°C. per minute). The glass transition temperature was 218° C.

EXAMPLE 18

Following the procedure in Example 17, an unreinforced casting wasprepared from 98 g of the Epiclon 830/polysulfone/Cyanate Ester Bmasterblend (18/8/72 wt. ratio) and 6.0 g of 1,1-dimethyl-3-phenyl urea.The glass transition temperature on the cured casting was 218° C.

Control H

An unreinforced casting was prepared using Epiclon830/polysulfone/Cyanate Ester B masterblend (18/8/72 wt. ratio). Theprocedure was the same as in Examples 17 and 18 except that no ureacompound was used as an accelerator. The unreinforced casting had aglass transition temperature of 133° C.

Table II summarizes the glass transition temperatures of several cyanateester/accelerator formulations.

                  TABLE II                                                        ______________________________________                                        Cured Castings                                                                Ex.   Urea                       Tg                                           No.   Cmpd                       °C.                                   ______________________________________                                        Cyanate Ester A (100 pbw)                                                     12    1,1-dimethyl-3-phenyl urea (4 pbw)                                                                       197                                          13    1,1-dimethyl-3-phenyl urea (6.7 pbw)                                                                     218                                          14    1-(4-chlorophenyl)-3-(3,4-dichlorophenyl) urea                                                           216                                                (4 pbw)                                                                 E     None                       124                                          Cyanate Ester C (100 pbw)                                                     15    1-(4-chlorophenyl)-3-(3,4-dichlorophenyl) urea                                                           195                                                (4 pbw)                                                                 F     None                       88                                           Cyanate Ester A (80 pbw) and Epoxy DEN 431 (20 pbw)                           16    1-(4-chlorophenyl)-3-(3,4-dichlorophenyl) urea                                                           245                                                (4 pbw)                                                                 G     None                       233                                          Cyanate Ester B (72 pbw), Epoxy Epiclon 830 (18 pbw)                          and PSF (8 pbw)                                                               17    1,1-dimethyl-3-phenyl urea 218                                                (2 pbw)                                                                 18    1,1-dimethyl-3-phenyl urea 218                                                (6.1 pbw)                                                               H     None                       133                                          ______________________________________                                         Note: Cure schedules and conditions are shown in text.                   

The Tg data for the various unreinforced castings summarized in Table IIdemonstrate the high degree of cure obtained with the compositions ofthis invention under standard cure schedules commonly used in the art.Equivalent formulations without a cure accelerator achieved a much lowerdegree of cure when subjected to equivalent cure schedules.

EXAMPLE 19

A 500 ml 3-necked flask equipped with a paddle stirrer, thermometer,inlet and outlet for inert gas, and an electric heating mantle wascharged with 200 g of Cyanate Ester A. The resin was heated withstirring to 90° C. at which temperature a mixture of 8.0 g of DEN 431epoxy novolac resin and 8.0 g of 1,1-dimethyl-3-phenyl urea was added.The mixture was stirred for 1.5 hr at 100° C., degassed to removetrapped air bubbles, and then poured into a glass mold with dimensionsof 1/8"×8"×9". The resin was cured in a forced air oven using thefollowing cure schedule:

Heat from 77° F. to 248° F. in 1 hr

Hold at 248° F. for 1 hr

Heat from 248° F. to 350° F. in 1 hr

Hold at 350° F. for 2 hr

Cool from 350° F. to 77° F. in 1 hr

The cured casting was hard and transparent. Specimens were cut from thecasting to measure mechanical and thermal properties. The tensilestrength of the cured resin was measured according to ASTM D-638 (Type 1dogbone) and was found to be 10,500 psi. The tensile modulus was 465ksi. The glass transition temperature was 220° C.

EXAMPLE 20

A 16 oz. glass jar was charged with 243 g of Cyanate Ester A and 45 g ofa blend of Epoxy DEN 431 and PSF (3:1 by weight) and heated in a forcedair oven at 80° C. for 15 min. The jar was then removed from the ovenand the contents were stirred for 5 min. to ensure homogeneity beforeadding 12 g of 1-(4-chlorophenyl)-3-(3,4-dichlorophenyl) urea. The geltime at 350° C. was 9.3 min. The mixture was cast and cured as inExamples 12-16, to provide test specimens. The Tg, determined as before,was 253° C.

EXAMPLE 21

A 5 1 flask equipped with a paddle stirrer, thermometer, inlet andoutlet for inert gas and heated in an oil bath was charged with 2315 gof Cyanate Ester A, 92.5 g of Epoxy DEN 431, and 92.5 g of1,1-dimethyl-3-phenyl urea. The mixture was heated at 95° C. for 1.75hrs to adjust its viscosity for making prepreg.

A thin film of the resin was cast on a silicone coated release paper. Aunidirectional prepreg tape was made by transferring the resin from thecoated paper to a ribbon of carbon fiber under the action of heat andpressure in a prepreg machine. The final prepreg had fiber areal weightof 143 g/m², a resin content of 33.4 percent by weight, and a width of12 inches. The fiber used to make the tape was a pitch-based carbonfiber with a tensile strength of 260 ksi, a tensile modulus of 75 msi,density of 2.0 g/cc, and a filament count of 2,000 filaments per tow.When stored at room temperature, the prepreg retained tack and drapecharacteristics for more than 15 days, demonstrating that the cureaccelerators of this invention possess a low degree of room temperaturecure activity, i.e., good latency.

The unidirectional tape was laid up into an 8-ply quasi-isotropiclaminate with a (0, ±45, 90)_(s) configuration. The laminate was curedin a autoclave under a pressure of 100 psi at a temperature of 350° F.for 2 hrs. The cured laminate was rigid and hard.

EXAMPLE 22

A 2 1, 3-necked flask equipped with thermometer, inlet and outlet forinert gas, a paddle stirrer, and heated in an oil bath was charged with180 g of Epiclon 830 and 80 g of powdered polysulfone. The mixture washeated and stirred for 45 min at 140° C. as the polysulfone dissolved.To this stirred solution at 140° C. was added 720 g of Cyanate Ester Bover a 10 min period. As soon as the addition was complete, the mixturewas cooled with stirring to 100° C. Ten minutes later, when the mixturereached 100° C., 40 g of 1,1-dimethyl-3-phenyl urea was added. Themixture was heated and stirred at 100° C. for 15 minutes and thendischarged into pans. Another 1000 g resin batch of Epiclon830/polysulfone/Cyanate Ester B/1,1-dimethyl-3-phenyl urea was preparedin an identical manner. The two resin batches were combined, warmed to80° C., and coated as a thin film on release paper. A unidirectionalprepreg tape was prepared by transferring the resin from the coatedpaper to a ribbon of carbon fiber under the action of heat and pressurein a prepreg machine. The final prepreg had a fiber areal weight of 147g/m², resin content of 36.8 percent by weight, and nominal width of 12inches. The fiber used to make the tape was a polyacrylonitrile-basedcarbon fiber with a tensile strength of 730 ksi, tensile modulus of 41.5msi, density of 1.8 g/cc, yield of 0.44 g/m and filament count of 12,000per tow. The uncured prepreg tape, stored at room temperature, retainedtack and drape for more than 14 days, again demonstrating the excellentlatency of cyanate ester formulations that incorporate the urea cureaccelerators of this invention.

The unidirectional tape was laid up into a 32 ply, 15"×15" laminate withan orientation of (0, ±45, 90)_(4s). The laminate was cured in anautoclave under a pressure of 100 psi using a straight up cure cycle(ramp from 77° F. to 350° F. at 3° F./min; hold 2 hr at 350° F.; cool to77° F. at 3° F./min). The cured laminate was machined into 4"×6" testpanels. The panels had a nominal thickness of 0.18 inches and wereimpacted in the center with a Gardner type impact tester (GardnerLaboratories, Bethesda, Md.) having a 5/8 inch diameter sphericalindenter. The impact was normal to the plane of fibers. When impacted,the 4"×6" panel was simply supported over a 3 inch by 5 inch cutout inan aluminum plate with a plywood backup. The impacted panel was testedfor residual compressive strength in a steel fixture that constrainedthe edges from out-of-plane buckling. After an impact of 1500 in-lb/ inof thickness, the test panel had a residual compressive strength of 31ksi.

EXAMPLE 23

A 2 liter, 3-necked flask equipped with a paddle stirrer, thermometerand inlet and outlet for inert gas was charged with 860 g of CyanateEster B.

The flask was placed in an oil bath at 100° C. and the contents werestirred as 120 g of Epoxy DEN 431 was added. The resulting solution wasstirred at 100° C. for 10 minutes. Then 20 g of 1,1-dimethyl-3-phenylurea were added and agitation was continued for an additional 5 minutes,after which the resin was discharged into pans and cooled.

The urea-accelerated resin was warmed to 80° C. and coated as a thinfilm on silicone treated release paper in a strip about 7 inches wide. Aunidirectional tape was made by sandwiching a ribbon of carbon fiberbetween two layers of resin-coated release paper and then subjectingthat sandwich to heat and pressure in a prepreg machine. The prepregtape was 6 inches wide, had a fiber areal weight of 146 g/m² andcontained 30.2 weight percent resin. The fiber used to make the tape wasa polyacrylonitrile-based carbon fiber with a tensile strength of 730ksi, a tensile modulus of 41.5 msi, a yield of 0.44 g/m, and a filamentcount of 12,000 per tow.

The tape was laid up into a sixteen ply unidirectional laminate (6"×12")and cured in an autoclave using the following cure schedule:

Heat from 75° F. to 266° F. at 3° F./min.

Hold at 266° F. for 1 hr.

Heat from 266° F. to 350° F. at 3° F./min.

Hold at 350° F. for 4 hr.

Cool from 350° F. to 77° F. at 3° F./min.

The laminate was postcured in a forced air oven at 220° C. for 4 hr. Thepostcured laminate was machined into test coupons to measure 0° flexuralstrength according to ASTM D-790. When tested at 350° F., the laminatehad a flexural strength of 163 ksi, a flexural modulus of 21.7 msi and afiber volume fraction of 558.8 percent. Another coupon was immersed inwater at 160° F. for two weeks prior to testing. When tested at 350° F.,this sample had a flexural strength of 137 ksi and modulus of 198 msi,indicating good property retention at elevated temperature under wetconditions.

EXAMPLE 24

Prepreg tape made in Example 23 was laid up into a 6"×12" 10-plylaminate with a configuration of ((±30)₂ 90 )_(S). The laminate wascured and postcured as in Example 23 and then cut into 1"×10" strips foredge delamination strength testing. Edge delamination strength is ameasure of composite toughness. The details of the test are described inthe reference: SAMPE Journal Vol. 18, No. 4, July/August 1982, p 8 by T.K. O'Brien.

The edge delamination strength of the laminate was determined to be 32.2ksi.

EXAMPLE 25

Prepreg tape made in Example 23 was laid up into an 8-ply 6"×12"laminate with a (±45)_(2s) configuration. The laminate was cured andpost cured as in Example 23 and then cut into 1" by 10" strips tomeasure high temperature property retention in the wet condition. Thespecimens were immersed in water at 160° F. for 2 weeks and then placedin an Instron testing machine to determine stiffness. The stiffness intension of the moisture-conditioned specimen was determined at roomtemperature and at 350° F. after heating the specimen to thattemperature in less than one minute. In this test the moistureconditioned composite retained 62 percent of its room temperaturestiffness at 350° F., indicating excellent retention of stiffness atelevated temperature under wet conditions.

EXAMPLE 26

A 1 gallon working capacity Baker Perkins sigma blade mixer was chargedwith 4.0 Kg of Cyanate Ester A. The resin was heated to 140° C. and then1.0 Kg of powdered polysulfone was added. The cyanate ester polysulfonemixture was blended and heated at 140° C. until all the polysulfonedissolved (2.5 hr), then cooled and discharged from the mixer.

A 2.0 Kg portion of the cyanate ester/polysulfone solution was blendedwith an additional 2.0 Kg of Cyanate Ester A at 80° C. for 1 hour. Adispersion of 160 g of 1-(4-chlorophenyl)-3-(3,4-dichlorophenyl) urea in400 g Epoxy DEN 431 was added and mixing was continued for an additional30 min. before discharging and cooling the resin formulation.

A sample of the resin was placed in a Brookfield Thermosel viscometerand the viscosity of the mixture was measured as the temperature of theThermosel was raised at 1.1° C./min. The viscosity of the resin wasapproximately 100,000 cps at 70° C. The minimum viscosity was 2,500 cpsxat 135° C., after which the viscosity started to increase due to theeffect of the urea accelerator on the cure.

A thin film of the resin was cast onto a siliconecoated release paper. Aunidirectional prepreg tape was made by transferring the resin from thecoated paper to a ribbon of carbon fiber under the action of heat andpressure in a prepreg machine. The final prepreg had a fiber arealweight of 141 g/m², a resin content of 38.4 percent by weight, and awidth of 12 inches. The fiber used to make the tape was apolyacrylonitrile-based carbon fiber with a tensile strength of 350 ksi,tensile modulus of 57 msi, yield of 0.36 g/m, and filament count of6,000 per tow. The unidirectional tape made with this resin retainedtack and drape for more than 15 days.

The invention will thus be seen to be the use of organo urea compoundsas latent cure accelerators for cyanate esters, and thermosettingformulations comprising cyanate esters and urea compounds. The ureacompounds useful as latent cure accelerators in the practice of thisinvention have a plurality of organo radicals as N-substituents. Thesesubstituents may be aliphatic or aromatic hydrocarbyl radicals, such as,for example, alkyl, alkylene, aralkyl, aryl and the like. Thehydrocarbyl radicals may also include further substituent groups such ashalogen or the like that are not reactive with the remaining components.The cyanate ester formulations, which may further include such materialsas coreactants, stabilizers, fibers, pigments and the like as iscommonly practiced in the art, may be useful as adhesives, coatings,impregnating and laminating resins and encapsulating resins. While theinvention has been illustrated by means of various examples which areintended to be non-limiting, it will be apparent to those skilled in theart that further modifications and variations are possible withoutdeparting from the spirit and scope of the invention, which is definedby the appended claims.

We claim:
 1. A thermosetting composition comprising a cyanate ester anda urea compound having the structural formula RR¹ NCONR² R³ wherein Rand R² are independently selected from the group consisting of hydrogenand organo radicals, and R¹ and R³ are independently selected organoradicals.
 2. The composition of claim 1 wherein said organo radicals arehydrocarbyl radicals selected from the group consisting of alkyl,aralkyl and aryl radicals.
 3. A thermosetting composition comprising acyanate ester and a urea compound selected from the group consisting ofalkyl aryl ureas, aryl ureas and mixtures thereof.
 4. The composition ofclaim 3 wherein the urea compound is an alkyl aryl urea selected fromthe group consisting of 1,1-dimethyl-3-phenyl urea,1,1-dimethyl-3-(4-chlorophenyl) urea,1,1-dimethyl-3-(3,4-dichlorophenyl) urea, 2,4-bis-(N,N-dimethylureido)toluene, and mixtures thereof.
 5. The composition of claim 3 wherein theurea is a diaryl urea selected from the group consisting of 1,3-diphenylurea, 1-(4-chlorophenyl)-3-(3,4-dichlorophenyl) urea and mixturesthereof.
 6. A thermosetting composition comprising 100 pbw of a cyanateester and from about 0.5 to about 12 pbw of a urea compound selectedfrom the group consisting of alkyl aryl ureas and aryl ureas.
 7. Thecomposition of claim 6, wherein said urea compound is selected from thegroup consisting of from 1,1-dialkyl-3-aryl ureas and 1,3-diaryl ureas.8. The composition of claim 6 wherein said urea compound is selectedfrom the group consisting of 1,1-dimethyl-3-phenyl urea,1,1-dimethyl-3-(4-chlorophenyl) urea,1,1-dimethyl-3-(3,4-dichlorophenyl) urea, 2,4-bis(N,N-dimethylureido)toluene, 1,3-diphenyl urea, 1-(4-chlorophenyl)-3-(3,4-dichlorophenyl)urea and mixtures thereof.
 9. The composition of claim 8 wherein thecyanate ester is an aryl cyanate ester having a plurality of cyanategroups per molecule.